Okay, so here's something that's been bugging me for a while.
In Webmaster tools -> Diagnostics -> Web Crawl, I am getting a dozen or so 404 errors reported. The thing is, I can't find anything linking to these urls. This isn't just a fluke - it's been like this for a good 6 months or longer.
I have searched: Webmaster tools -> Pages with Internal Links Webmaster tools -> Pages with External Links Y! linkdomain: Xenu's sitespider
All report 0 links to all of these pages that webmaster tools is reporting 404's on.
Lots of people notice something like this. If you expand the "How do I use this data?" link inside a Webmaster Tools account says the following: "...if unrecognizable URLs appear in the 404 report, it's fine to ignore them."
Also note the following - which tells you Google may be asking for a url on your domain that they found elsewhere on the web:
If another site lists a broken link to your site, we may list that URL but you may not be able to fix the error.
I figured wasn't anything to worry about... just one of those little quirks that bugs ya a little. My best guess was that it was something that used to have a link in a dated datacenter.. and the spider was still trying to pull it.
Whenever I've seen those, it has turned out to be another site (often a messageboard) that linked to the wrong url on my site. Usually they're off by one letter. I add them to my 301 redirect. That way, if someone is on that other site and clicks on the link, they'll wind up on the intended page.